I find that the best responses are short and sweet -- "Yep, he's pretty quick!" and move on if at all possible. If they keep at it, I'm honest about the issues. After all, they asked! If they act jealous or say their child could be advanced too, I usually go with, "Oh, then you should ask that he be assessed!", nice and cheery. As it becomes clear that I'm not going to play the competitive game, most people let it rest.
As ds6's new school, all the kids are gifted, but grouped by ability. It's interesting to watch--everyone wants to know what the other kids are doing, but no one wants to ask! It's a self-imposed "don't ask don't tell" policy that works beautifully to quell competition. If only the world were like that!
